Liquid crystal display devices make good use of their characteristics, such as lightness, thinness, and lowness in consumption power to be very frequently used as flat panel displays. The use thereof has been spreading year after year for portable telephones, personal digital assistance (PDA), personal computers, televisions, and others.
However, liquid crystal display devices have a problem that their viewing angle is narrower than that of CRTs. Against this problem, Patent Document 1 suggests that a light diffusing film having a function of causing incident light to scatter/penetrate in/through the film is laid on a liquid crystal display screen. This film is yielded by melt-extruding, into a sheet form, a composition having an island-in-sea structure made of transparent resins different from each other in refractive index, and further drawing the sheet. Intensity distributions of scattering/penetrating light in the film, which are shown in FIGS. 3 and 4 of Patent Document 1, respectively, suggest that in each of these cases, an enhancement of the film in viewing angle and a decrease in the darkening are not compatible with each other.
Patent Document 2 suggests an anisotropic scattering spectroscopic film which has a distribution of a scattering angle varied in accordance with wavelength, and further has scattered light distributions different along two directions having different azimuth angles to the film plane by 90 degrees. This film produces a very good effect for an enhancement in viewing angle in the same way as in Patent Document 1. However, a scattered light distribution of a film that is shown in FIG. 3(b) of Patent Document 2 suggests that a decrease in the darkening is still insufficient.
Furthermore, Patent Document 3 suggests a transmitted-light-scattering controlling film which is made of a single thermoplastic resin, and has a region containing many microscopic pores inside of the film. This film makes use of light-scattering based on a matter that polycarbonate is melted into the form of a film and the film is drawn so that cracks in the form of trenches are generated. However, from the angle-dependency of the light scattering intensity of the film in FIG. 13 of Patent Document 3, it is suggested that the film is insufficient in diffusivity and the darkening is satisfactorily decreased, but the effect of enhancing the viewing angle is insufficient.
Furthermore, Patent Document 4 suggests a light scattering sheet yielded by mixing polymethyl methacrylate PMMA with styrene/acrylonitrile copolymer SAN with each other, melting the mixture, casting the melted mixture, and drying the mixture. This film has a phase-separated structure composed of plural resins different from each other in refractive index, and further has a co-continuous phase structure formed by spinodal decomposition. However, from the angle-dependency of the light scattering intensity of the film in FIG. 4 of Patent Document 4, it is suggested that the film is large in diffusivity so that the effect of decreasing the darkening is insufficient. As described in Patent Document 4 also, the effect of this film is only an effect of uniformalizing brightness which depends on a change in the viewing angle.
Additionally, disclosed are many techniques using a lens film. About, for example, a technique disclosed in Patent Document 5, from diffusivity distributions of transmitted light of a film in FIGS. 8 and 9, the same matter as in Patent Document 1 is suggested. In other words, in the direction in FIG. 8, the film is high in diffusivity so that the film is excellent in viewing-angle-enhancing effect although the darkening is poorly decreased while in the direction in FIG. 9, the film is low in diffusivity so that the darkening is satisfactorily decreased although the film is poor in viewing-angle-enhancing effect.
As described above, light diffusing films of liquid crystal display devices in the prior art satisfy either an enhancement in viewing angle, or a decrease in the darkening. However, under the present circumstances, a film which attains the two properties at a high level has not been created yet.
The display screen of liquid crystal display devices has a problem of being easily stained, or scratched (or injured). Thus, disclosed is a protective film, for a liquid crystal display device, which prevents the display screen of the device from being broken, stained, scratched, or damaged (for example, Patent Document 6).
Disclosed is also a protective film having the so-called peep preventing function, which is a film set to the front surface of a liquid crystal display device to control the visible range of its display screen in such a manner that an image thereon cannot be viewed diagonally from the front of the screen although the image can be viewed from substantially the front (for example, Patent Document 7).
As described above, liquid crystal display devices have a problem that their viewing angle is narrower than that of CRTs. For example, a car navigation system is set up in a direction diagonal to a driver in many cases. Thus, the viewing angle thereof is desired to be enhanced. For example, as functions for display information of a portable telephone are made into a higher level, or some other progress is further made, a use manner such that its display screen is viewed by many persons is increasing. Thus, an enhancement in the viewing angle (of the screen), which is reverse to the peep-preventing function, is being intensely required. In light of such a background, a protective film, for a liquid crystal display device, to which a viewing-angle-enhancing function is added is intensely desired.